1. Field of the Invention
The present invention relates to multi-chip modules and more particularly to a structure providing improved coplanarity accommodation and heat dissipation in a multi-chip module.
2. Background Art
Ser. No. 10/620,029 and Ser. No. 60/576,766 disclose several embodiments of a multi-chip module (MCM), including a conductive element which serves as an electrical connector for electrically connecting respective electrical contacts on at least two power semiconductor devices. In addition to serving as an input or output connector, the conductive element improves heat transfer from the power semiconductor devices through the top of the module. This type of MCM is referred to commercially as a T-PAC™ module by its manufacturer, the International Rectifier Corporation.
One of those embodiments is the T-PAC module 54 shown in FIG. 1. A conductive element 56 bridges over and electrically connects respective contacts on a conventional MOSFET 30 and a flip-chip MOSFET 42 with its web portion 60. The conductive element 56 may be made of copper or another heat-conductive metal such as Au, Ag, Sn, Ti, brass, Ni or Al. A connector 64 connects the web portion 60 to a printed circuit board 40. The connector 64 may optionally be disposed between the two MOSFETS 30, 42 rather than in the position shown.
In more detail, a top surface of the web portion 60 of the conductive element 56 is exposed to the exterior of the T-PAC module 54, which improves heat dissipation from the MOSFETs 30, 42 through the top of the module. The above components are placed in a mold and resin is injected, forming a molded housing 58, disposed around and between the conductive element 56, its connector 64, the conventional MOSFET 30, and the flip-chip MOSFET 42. The resin is not, however, molded over the top of the conductive element 56.
A typical layout of an MCM (multi-chip module) 55 is shown in plan view in FIG. 2. In this example, the MCM 55 contains only the one T-PAC module 54. As shown, additional components C1, C2, C3 and C4 may be disposed on the circuit board along with the T-PAC module 54 and within the molding 58. Additional layouts are shown in Ser. No. 60/576,766.
A problem in this technology is that the components within an MCM module, which may include one or more T-PAC module(s), may differ in height. The presence of a tall T-PAC module in the mold may cause resin to flow in the mold over the top of a shorter T-PAC module, which is undesirable since the tops of the T-PAC modules should remain exposed.
FIG. 3 is a schematic cross-sectional view of the MCM 55 in FIG. 2 as seen during an intermediate manufacturing step. A T-PAC module 54 containing components such as switching components, passive components or MOSFETs as seen for example in FIG. 1 is disposed at the center of the printed circuit board 40, while other components C1 and C4 are disposed on other portions of the printed circuit board 40.
A lead frame or another type of substrate may be substituted for the printed circuit board 40.
Before forming the molding 58, a compliant plastic film 70 is applied over the tops of the module 54, the component C1 and the component C4, thus sealing off the tops of the components while compensating for the height differences between the respective components. The film 70 prevents mold compound from flowing over the top of any component. The film may be any high temperature plastic film, such as Kapton. The resin molding material 58 is applied thereafter, after which the film 70 is removed.
FIGS. 4, 5 and 6 show the molding process. In this example, three T-PAC modules 54, 54a, 54b are disposed on the board 40. The film 70 is pressed onto the assembly of the modules and the board by tools 62, 64, and more particularly by a spring-loaded plate 66 on the tool 62, while the plastic molding compound is injected around the modules.
This molding arrangement successfully prevents mold compound from flowing over the tops of the components. However, it has been found that if the top surface of the conductive element 56 of a T-PAC 54 is excessively high above the printed circuit board 40, excess film material may spill over and form protrusions 57 at the sides of the conductive element, as shown in FIG. 3. Then, when the molding material 58 is added, these protrusions may prevent the resin material from reaching the conductive element, forming a void between the material and the conductive element which reduces the structural integrity and strength of the MCM and its resistance to environmental hazards.